I had the exact same thing on the inside of my hand before ... it only hurt if I put pressure on it, and I could see the bump if I pulled my finger back. I had it for a couple years and then one day it was completely gone.

Do you feel that the lump is between your skin and your bone, or is it attached to the bone?

It it mobile or fixed?

About how big is it?

Is it hard or soft?

It sounds like it could be a cyst, an encondroma (this would be at least partially inside the bone), or some sort of tendon swelling (you're too young to have Rheumatoid arthritis, so I dunno what this could be).